1. Field of the Invention
This invention relates to a butterfly valve comprising a valve body having a fluid passage, and a valve disc rotatable about an axis perpendicular or about perpendicular to a center line of the fluid passage, wherein a hard annular disc seat defined on the periphery of the valve disc is adapted to contact a hard annular body seat provided in the valve body over the entire circumference thereof when the valve disc is in a position perpendicular or about perpendicular to the center line of the fluid passage, and the body seat has a sealing surface corresponding to the peripheral surface of an imaginary cylinder having an axis inclined at an angle to the center line of the fluid passage in the direction of rotation to a closed position of the valve disc.
2. Description of the Prior Art
This type of valve has both the body seat in the valve body and the disc seat defined on the valve disc made of hard material that is free from change in quality over a long period of time, corrosion-resistant, mechanically strong, and invulnerable to wearing action of sand and the like contained in fluids, hence the body seat and the disc seat have long life. Since the valve disc assumes a position normal to the center line of the fluid passage to close it, the valve disc has the shape of a disc, which means an advantage of easiness in manufacture and shaping.
Furthermore, the cylindrical surface of the body seat which contacts the disc seat has the advantage that it is easy to finish such sealing surface of the body seat with high precision by means of machine tools in wide use.
In checking fluid leak through the body seat and the disc seat placed in contact with each other in an inclined manner, an internal fluid pressure, that occurs as the valve is closed, acts to expand the valve body, and this is followed by a rotation of the valve disc twisting the valve stem under the fluid pressure. This provides a mutual relation between the body seat and the valve disc wherein the former securely surrounds the latter over the entire circumference, and an excellent sealing condition is attained as a result.
The following observation is made of the interrelation between the angle of inclination and a further rotation to a very slight extent of the valve disc after closing. Where the angle is large, the ratio of a tangential force, namely a torque, acting on the contacting surfaces to a vertical force, namely a surface pressure, tends to be large, and the closing strength by the further rotation of the valve disc cannot surpass the friction between the sealing surfaces. Therefore, fluid leak occurs even under a relatively low internal pressure. In order to provide a good sealing condition, a certain degree of design surface pressure is essential to eliminate minute clearances between the two surfaces as much as possible. Conversely, where the angle is small, the valve disc makes the further rotation easily to provide a good sealing condition even under a high pressure. However, in the event that such further rotation is somewhat excessive, a different problem arises.
That is, the internal fluid pressure acts on one side of the valve disc when it is closed and the valve is opened after the pressure has been removed when occasion demands. If in such a case the further rotation of the valve disc is excessove at the time of applying or reducing pressure, the valve body contracts upon removal of the pressure while the valve disc remains in a closed position further advanced from the initial closed position, such that a large diameter portion of the valve disc is pressed into the body seat having a smaller diameter. This requires a greater driving force for opening the valve than for closing it, and sometimes it is impossible to open the valve without applying such a great torque as would damage the surfaces of the body seat and the disc seat.
In case the angle of inclination is smaller still, the valve disc could move beyond the dead point for full closure and then it could be opened only by expanding the valve body with the valve disc.